1. Field of the Invention
The present invention relates to a method and system for continuously providing a dry material and liquid slurry. For example, a plaster and water slurry used in the production of plaster molds in the ceramics industry.
2. Description of the Prior Art
Presently in use in the various industries utilizing plaster molds is the apparatus and method disclosed by U.S. Pat. No. 3,006,615. This U.S. Patent disclosed an apparatus which provides for mechanical mixing of metered dry plaster and metered water in a mixing chamber to form a continuous slurry. The slurry, thus formed, is pumped by a slurry pump to the place where the slurry is poured into case molds, flasks or the like and when hardened the slurry forms plaster molds.
However, lengthy experience with the equipment, as disclosed in U.S. Pat. No. 3,006,615, indicates certain problems have arisen in specific situations. The principle problem appears to reside with the step of mechanical mixing as disclosed by U.S. Pat. No. 3,006,615. The mechanical mixing appears to produce slurry exhibiting undesirable characteristics when said slurry is formed into plaster molds used in ceramic casting for the hobby mold industry and for the ceramic tableware or china dinnerware industry.
After study it has been concluded that the mechanical means used to mix the dry plaster and water into a slurry causes the mixture to harden on the mixing element and on the surfaces of the mixing chamber and that a certain scraping of the surfaces of such set plaster inevitably occurs. It is this scraping and grinding action which strews hardened plaster particles (known as terra alba) into the plaster slurry being generated and causes a deleterious effect in the crystalline structure of the resulting plaster molds.
In order for one to appreciate the significance of terra alba in the slurry it will be necessary to give a brief description of the chemical structure of gypsum which is the basic substance of plaster. Gypsum chemically is calcium sulfate and when it is in the dry powdered state as received by a mold shop it is in a form known as hemi-hydrate or CaSO.sub.4.1/2H.sub.2 O. This is the uncrystallized gypsum as it exists following calcining in the gypsum production mill--one half molecule of water exists in chemical combination with the CaSO.sub.4. When additional mixing water and hemi-hydrate gypsum are brought together another one and one half molecules of water begin to combine with the hemi-hydrate to produce CaSO.sub.4 --2H.sub.2 O designated as gypsum dihydrate. At this point the gypsum has solidified into crystals. Although there is excess water present beyond that needed for the chemical reaction, such excess water remaining in the molds is normally dried off in air circulation rooms or in heat driers.
There are in the setting of plaster (the formation of gypsum crystals CaSO.sub.4.2H.sub.2 O) many factors to consider but one in particular is pertinent to the disclosure of this invention. That factor is mechanical mixing of the plaster slurry and its effect on crystal formation. Crystallography photo micrographs reveal that gypsum crystals in molds produced by a continuous mixer as disclosed in U.S. Pat. No. 3,006,615 are on average less randomly oriented and more frequently broader and shorter and more uniformly distributed than in molds produced by the conventional batch method of stirring such slurry in a pail. The effect of rapid mechanical mixing which a continuous production system entails, creates the particles of terra alba previously described and it is well known in the art that terra alba is an accelerator for setting plaster. What occurs is that the terra alba seeds the crystal growth in the plaster slurry causing the crystals to grow fast and early. Subsequently attrition of these crystals by additional mixing breaks them down so that the final result is a relatively uniform matrix of gypsum crystals having few random large crystal clumps or excrescenses. In the molds used by the ceramic industry this can be of significant importance because a high degree of mold absorption is important for good casting and randomly oriented, non-uniform crystals yield molds of greater relative absorption, while more uniform crystals more evenly dispersed, yield molds of lesser absorption. Obviously any continuous mixing apparatus which utilizes a high speed mixing element and a mixing chamber to produce a plaster slurry will always exhibit the aforementioned terra alba effect, albeit in greater or lesser degree depending on how much terra alba is strewn into the slurry. Especially at a very low throughput of slurry through the mixing chamber (where an essentially constant quantity of terra alba is being generated) there will be a pronounced effect in the crystalline structure and consequently reduced absorbency of the resultant molds.
In the ceramic industry hobby molds and molds used for china tableware are relatively small. In consequence their production requires a relatively low throughput of plaster slurry and inasmuch as the terra alba effect is greatest when the throughput is lowest the result in these industries has been widespread lack of success with continuous mixing.